As for a conventional color image processing device 100 of this kind, e.g. a color printer, digital separated color signals are produced for the r (red), g (green), and b (blue) colors as a result of a scanner 102 reading a color original 101 by optical scanning, for example, as shown in FIG. 4. The separated color signals are corrected by .gamma.-correction means 103 for level, non-linearity, etc. and transmitted to color adjusting means 104.
As will be detailed later, the separated color signals are chromatically adjusted and converted into separated color signals for the C (cyan), M (magenta), and Y (yellow) colors by the color adjusting means 104, before being transmitted to a printer section 105 for a print-out.
The color adjusting means 104 is provided as above for following reasons.
The characteristics of the scanner 102 and the printer section 105 vary greatly from one model to another. If the separated color signals produced by the scanning by the scanner 102 are converted into the colors of C, M, and Y for a print-out without being chromatically adjusted, the copied image has different colors from those of the original. The color adjusting means 104 performs color adjustment so that original colors can be reproduced precisely.
Conventionally, it has been suggested that the following three-dimensional look-up table, as an example, is to be stored as data in a memory device 106 for the use in the color adjustment by the color adjusting means 104.
The three-dimensional look-up table is produced in the following manner. Data for producing color samples of, for example, 9.times.9.times.9=729 colors, which are combinations of the values of C, M, and Y, is produced by a device (not shown) so that those colors available for output are chromatically adjusted uniformly. The data is then outputted to the printer section 105 to produce color samples. Subsequently, the color samples are read by the scanner 102 to derive separated color signals. The relations between the r, g, and b values of the separated color signals and the C, M, and Y values of the data for producing the color samples are stored in the memory device 106 as the data on the separated color signals for C, M, and Y corresponding to those for r, g, and b in the form of a three-dimensional look-up table.
More specifically, the three-dimensional look-up table is configured as shown in FIG. 5(a): The inputted separated color signals for r, g, and b are located as three-dimensional space coordinates. The separated color signal coordinates are divided into unitary cubes. Optimum separated color signals to be outputted then become related to vertex coordinates of the unitary cubes.
In such a case, for a reduction in data volume to be stored, the correspondence table for the data on the separated color signals and the color adjusted signals deals with a limited number of colors. As for the input of separated color signals that cannot be found in the three-dimensional color look-up table, interpolative calculations are done on the three-dimensional data on the basis of the known color adjusted signals found in a neighborhood of the inputted separated color signals.
Specifically, as shown in FIG. 5(b), the interpolative calculation described in the following formula (1) is performed on the data on the eight vertexes of the unitary cube to which the data belongs: ##EQU1## where Dx represents a color adjusted signal, Dxi represents a known color adjusted signal, and Vi represents a volume of a rectangular parallelopiped located at the diagonal angle from the vertex.
The environment may affect the printer section 105 and causes aging in the characteristics of the device, which leads to printed results of undesirable colors.
U.S. Pat. No. 5614934, as an example, discloses a conventional technology that enables the printer to stably print desired colors regardless of such aging.
To be more specific, as shown in FIG. 4, a color sensor 109 and a feedback control section 107 are always provided. Every time a predetermined period of time elapses, color data acting as a reference is produced and outputted to the printer section 105 by a reference data producing section 108 for a print-out. The reference color derived in this manner are separated into color signals and read by the color sensor 109. The color signals are inputted to the feedback control section 107. The contents of the three-dimensional look-up table stored in the memory device 106 are altered on the basis of the color signal data that is read and the color data produced by the reference data producing section 108.
However, the color image processing device 100 having the conventional arrangement shown in FIG. 4 needs to be provided with, apart from the color adjusting means 104 and the memory device 106, those unique circuits 107, 108 and 109 to deal with aging, which makes the device more complex as a whole. Besides, in order to deal with aging, the user needs to perform the above operation regularly by him/herself, which is troublesome.